When the scene is run directly, I can add the view in the scene's setup() method to the scene's view property. However, if the scene is run as a modal scene, its view property is None.

That's not too big a deal. I can just run the modal scene and then trigger the ui.View to be added after it's being presented. But where should the ui.View be added? I can add it to the parent scene's view, but then it doesn't disappear when the modal scene is dismissed.